The present invention relates to a device for positioning a photolithography mask with respect to the surface of a wafer, in order to carry out operations of exposure of the wafer or substrate, in contact/proximity mode photolithography in particular.
It also relates to an appliance comprising such a device, and a method implemented in this device or this appliance.
The field of the invention is more particularly, but non-limitatively, that of steppers and exposure systems in photolithography.
Photolithography techniques require operations for the exposure of the wafers being processed.
The wafer to be processed comprises a layer to be etched, covered with a photosensitive layer called the “resist” layer. A mask with transparent portions and opaque portions is positioned above the surface of the wafer. Then the surface of the wafer is illuminated or exposed through the mask with a light generally within the ultraviolet (UV) wavelengths. In the transparent portions of the mask, the light reaches the wafer and changes the properties of the photosensitive layer. After a step of chemical treatment, the portions of the resist layer situated in the exposed areas or in the protected areas according to the process (positive or negative resin), are removed in order to allow the selective etching of the uncovered portions of the layer to be etched.
These operations may be repeated many times during a process. The mask must therefore be positioned with great accuracy with respect to the surface of the wafer, so that the optical path of the exposure rays is perpendicular and equal over the entire wafer or substrate, which generally comprises already-existing structures.
The positioning in the plane X-Y (parallel to the plane of the wafer) is generally carried out by superimposing patterns (reticles, crosses, etc.) present on the mask with patterns already etched on the surface of the wafer. The mask must be positioned at a distance that allows this alignment to be carried out without affecting the photosensitive resin layer by contact.
The mask must then also be positioned at a distance Z that is constant and very accurate with respect to the surface of the wafer, and this must be done over its entire surface. This makes it possible to control the diffraction of the light which passes through the mask. In fact this diffraction directly determines the accuracy and the resolution with which the patterns of the mask can be reproduced on the wafer. Controlling this distance Z is even more important when short wavelengths (UV or Deep UV or Extreme UV EUV) are utilized in order to limit with precision the effects of diffraction and maximize the spatial resolution of the etching.
The mask must not be in contact with the wafer. It is generally positioned at a distance Z of the order of 20 micrometers or less.
In the known contact/proximity exposure systems, the positioning of the masks is carried out with microbeads or cylinders, made from ceramic material, with a calibrated diameter, which serve as shims. These microbeads or shims are firmly fixed to mobile elements making it possible to insert them between the mask and the wafer, after which the assembly is pressed.
This technique has a certain number of drawbacks:                it leads to complex systems;        the mechanical contact may degrade the contact surfaces;        controlling the positioning of the mask in the plane X-Y and over distance Z is complex, because the surfaces must not be pressed on either side of the spacers until the positioning X-Y has been carried out;        the accuracy of the thickness control is not always sufficient;        it is not possible to reduce the distance Z below a certain limit, which is however desirable for limiting the diffraction.        
A subject of the present invention is to propose a device and a method for positioning a mask with respect to a wafer to be exposed which allows the drawbacks of the prior art to be overcome.
Another subject of the present invention is to propose a device and a method for positioning a mask with respect to a wafer or a substrate to be exposed which allows positioning without mechanical contact with the surface of the wafer.
Finally a subject of the present invention is to propose a device and a method for positioning a mask with respect to a wafer or a substrate to be exposed which allows accurate positioning of the mask at a very short distance from the wafer.